tty: Fix race in the flush for some ldiscs

If you issue an ioctl to flush a tty as the line discipline is changing or
otherwise unplugged you can get a crash. The bug is very old but the rest
of the BKL lock dropping and some very "good" luck on Ingo's part caught
an example.

Use the correct ldisc_ref form so that we wait for the ldisc change to
complete and then flush

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/drivers/char/tty_ioctl.c b/drivers/char/tty_ioctl.c
index a408c8e487ec006147078f2f7d22b6bce88925bb..6f4c7d0a53bf36bc6ff28afeadca4720704e459f 100644 (file)
--- a/drivers/char/tty_ioctl.c
+++ b/drivers/char/tty_ioctl.c
@@ -1057,7 +1057,7 @@ int tty_perform_flush(struct tty_struct *tty, unsigned long arg)
        if (retval)
                return retval;
 
-       ld = tty_ldisc_ref(tty);
+       ld = tty_ldisc_ref_wait(tty);
        switch (arg) {
        case TCIFLUSH:
                if (ld && ld->ops->flush_buffer)